The present invention relates to an optical network for telecommunication and an amplifier node which may be used in such a network. More particularly, it relates to a network and a network node for wavelength division multiplex transmission.
Such an optical network is generally formed of a plurality of nodes which are interconnected by optical fibres on which communication signals are transmitted as a wavelength division multiplex, i.e. modulated onto a plurality of carrier waves of different wavelengths which propagate simultaneously in the fibre.
Due to the attenuation of the carrier waves, communication signals that are to be transmitted over wide distances must be re-amplified at regular intervals. This reamplification may be done in nodes which are specifically provided for this purpose between two fibre sections, but the nodes may also interconnect a larger number of fibre sections and accomplish a switching function, i.e. they may separate an incoming wavelength division multiplex into its payload channels corresponding to different carrier wavelengths and forward these to various outgoing fibre sections.
The present invention relates to amplifier nodes of both types.
It is common to transmit on an optical fibre not only the payload channels, i.e. the carrier waves which convey payload data between terminals of the network, but also a so-called optical supervisory channel OSC, which conveys information required for controlling the payload channels and the information conveyed in them within the nodes of the communication network.
The information transmitted on the supervisory channel is purely internal information of the network which is not transmitted to terminals connected to the network and which may therefore use other transmission formats than the payload channels and is processed independently from the pay-load channels in the nodes of the network.
In most conventional networks for wavelength division multiplex transmission, a demultiplexer for demultiplexing the incoming wavelength division multiplex into the payload channels and a supervisory channel is provided immediately at the entry port of the node. The processing of the payload data channels and of the supervisory channel within the node is carried out completely separately from one another before reaching a multiplexer immediately before the exit port of the network node. The multiplexer reassembles the payload data channels and the supervisory channel into a wavelength division multiplex.
A disadvantage of this conventional design is that insertion losses caused by the use of the demultiplexer and the multiplexer attenuate the incoming wavelength multiplex at the location of its transmission path where it is weakest and, respectively, attenuate it even before it enters the transmission fibre. In order to compensate the insertion losses and to have sufficient signal power for further processing at the output of the demultiplexer, it would be conceivable to increase the transmission power fed into the optical fibre. However, this fails in most cases, because most networks already use so high a transmission power that a further increase would cause a significant increase of non-linear effects, which deteriorate the signals. The only possibility is, therefore, to reduce the distance between two amplifier nodes.
In U.S. Pat. No. 6,411,407, an amplifier node and an optical network, respectively, according to the pre-characterizing portions of the independent claims are proposed. In these amplifier nodes, there is provided a pre-amplifier between the entry port and the demultiplexer, and a post-amplifier between the multiplexer and the exit port, respectively, which overcompensate the insertion losses of the demultiplexer and the multiplexer. The carrier wavelengths of the payload data channels are distributed in a wavelength range of 1530 to 1560 nm, which is common in this field of technology, and which corresponds to that wavelength range in which the amplification of an erbium-doped fibre amplifier is independent of the wavelength. The supervisory channels are placed outside of this wavelength range, at wavelengths where the erbium-doped fibre amplifier has less amplification or no amplification at all. A supervisory channel which is branched off in the demultiplexer after passing through the pre-amplifier is terminated in the node and is generated anew at the exit side thereof, is combined with the outgoing payload channels in the multiplexer, passes a post-amplifier together with these and is transmitted on an outgoing optical fibre. With the use of the pre-amplifier it is possible to feed an incoming wavelength division multiplex into the demultiplexer at sufficient power so that at the output side thereof, sufficient signal power for further processing is available, and due to the placement of the post-amplifier behind the multiplexer, the output power of the post-amplifier is available without loss for feeding into the outgoing fibre, but for this, a decrease of reliability has to be accepted. If one of the two amplifiers at the beginning and the end of a transmission fibre fails, this not only prevents transmission of the payload channels, but also the available power of the supervisory channel at the receiver node decreases, so that it can no longer be reliably processed, whereby the detection of the failure and its causes as well as the repair thereof is made considerably more difficult, if not impossible.